DESCRIPTION: This is a new proposal which is a logical extension of the PI's past work describing clinically important antibodies to factor VIII in hemophiliacs. Human antibodies that inactivate blood coagulation factor VIII (FVIII inhibitors) develop in up to 37% of repeatedly transfused patients with severe hemophilia A. Although the effects of inhibitors are variable, significant bleeding and disability are frequent, and they are responsible for a large proportion of the cost of hemophilia care. There is no generally satisfactory approach to this problem. Detailed knowledge of the characteristics of FVIII specific T and B cells as well as the spectrum of the anti-FVIII antibodies is essential for the establishment of in vitro and in vivo studies of the immune response to FVIII and how it might be eliminated. The long term goal is to identify all anti-FVIII antibodies in hemophilic inhibitor patients. During the past 6 years, the applicant have pioneered the use of recombinant FVIII-derived polypeptides to localize the epitopes and to identify the inhibitory functions of anti-FVIII antibodies. They found that most patient plasmas contain anti-A2 and anti-C2 domain antibodies which are inhibitors of FVIII activity, and we have determined their mechanisms of FVIII inactivation. Their data from inhibitor neutralization assays suggest that as few as 3 different antibodies may account for the inhibitor titer in >90% of patients. They will determine if there are human antibodies which bind to the remaining FVIII domains A1, B, A3, and C1, if they inhibit FVIII activity, and what their mechanisms of inactivation are. The epitopes of the major inhibitors will be further localized. This information may suggest how mutant FVIII proteins which are fully active but unresponsive to inhibitors can be designed for patient therapy. They have developed immunoprecipitation assays which can detect anti-FVIII antibody levels 100-fold lower that those that are found in patients who have clinically significant inhibitors. The early immune response to FVIII in infant hemophiliacs will be examined to establish if one can predict which ones are tolerant and which ones will form antibodies to infused FVIII. It may be beneficial to initiate treatment to eradicate the anti-FVIII antibodies before they become a clinical problem. This possibility was suggested by a large, international study in which tolerance induction was most successful when it was begun while the inhibitor titer was low. For those patients who become tolerant after appearance of an inhibitor, it will be determined if this is due to antibody disappearance and/or to the appearance of additional noninhibitory antibodies. The presence of new antibodies in tolerant patients has been previously reported but their relationship to the tolerant state was not elucidated.